This invention relates to novel methods for the photodimerization and functionalization of various organic compounds, including alkanes, ethers, primary alcohols and silanes.
The phenomenon of .sup.3 P.sub.1 Hg photosensitization of alkanes and simple oxygenated compounds has been known and studied for the better part of this century. See Steacie, E.W.R., Chem. Rev. 22, 311, 1938; Cvetanovic, R. J., Progress in Reaction Kinetics 2, pp. 39-77, 1964; Strausz, O.P., Gunning, H. E. JACS 95, 746, 1973. Such investigations have revealed that alkane, ether or methanol vapor mixed with Hg vapor and photolyzed at 254 nm. exhibit the following basic behavior: ##STR1##
The patent literature is also replete with patents relating to the photochemical production of a variety of chemical compounds. For example, U.S. Pat. No. 1,961,493 (Hillis) describes the polymerization of gaseous olefins. U.S. Pat. No. 2,636,853 (Franklin) discloses the alkylation of C.sub. 3 to C.sub. 5 paraffins with like olefins. U.S. Pat. No. 2,640,023 (Cier) describes the production of branched paraffinic hydrocarbons from a mixture of saturated hydrocarbons. U.S. Pat. No. 2,655,474 (Schutze et al.) describes the reaction of at least two saturated hydrocarbons to form a branched product. U.S. Pat. No. 2,657,985 (Schutze et al.) also describes the production of a product comprising saturated branched chain hydrocarbons from a mixture of at least two saturated hydrocarbons. U.S. Pat. No. 2,730,495 (Gray) describes the production of alkyl and cycloalkyl hydroperoxides from a gaseous mixture of oxygen and one or more volatile paraffins or cycloparaffins. U.S. Pat. No. 2,762,768 (Cier) discloses the conversion of paraffinic hydrocarbons to a mixture of higher molecular weight hydrocarbons. U.S. Pat. No. 2,832,016 (Cier et al.) describes a reactor for conducting such a reaction. U.S. Pat. No. 2,908,622 (Bates) describes the photochemical production of formaldehyde from methane and oxide in the presence of mercury vapor and ultraviolet light. U.S. Pat. No. 2,976,422 (Hill et al.) discloses an apparatus for irradiating chemical reactants with gamma rays, including the polymerization of paraffins and isoparaffins. U.S. Pat. No. 3,083,152 (Folkins) discloses the conversion of saturated hydrocarbons of 2 to 9 carbon atoms to the corresponding unsaturated hydrocarbons by radiation at very high temperatures. No provision for the condensation of the reaction product within the reaction zone as they are formed is contemplated by the cited references.
Other patents which illustrate the state of the art regarding photochemical processes for treating organic compounds are U.S. Pat. No. 2,606,867 (Pianfetti et al.); U.S. Pat. No. 3,203,886 (Griffin); U.S. Pat. No. 3,384,658 (McCracken et al.); and U.S. Pat. No. 3,457,154 (Lester).
The following is the general consensus found in the literature (Strausz, 1973, supra; Cramer, W.A., J. Phys. Chem., 71, 1112, 1967) for the mechanism of alkane decomposition: EQU Hg+hv.fwdarw.Hg* [4] EQU Hg*+R--H.fwdarw.R+H+Hg [5] EQU H+R--H.fwdarw.H.sub.2 +R [6] EQU 2R.fwdarw.R.sub.2 [7] EQU 2R.fwdarw.R--H+OL [8] EQU H+OL .fwdarw.R [9]
More recent scrutiny has shown this scheme to be overly simplistic but has agreed in principle with this earlier consensus. In his 1973 publication, Strausz states that the .sup.3 P.sub.1 Hg atom sensitized decomposition of paraffins can be considered as a simple hydrogen atom transfer reaction. Strausz, 1973, supra.
This focus on a presumed free radical mechanism, the persistent adoption of the assumption that the Hg atom in this system is simply an energy transfer agent and not a catalyst, and a lack of interest in chemical synthesis caused these investigators to overlook the tremendous synthetic potential of the mercury photosensitization process. Although the patent literature describes reactions which produce products of practical utility, no reference was found in the extensive scientific literature which even mentions a potential commercial application for this process.
In our investigation of this process, we did not assume a free-radical mechanism and instead looked for signs of catalytic organometallic chemistry in this system involving R--M and M--H bonds. As a result, very persuasive evidence was accumulated which ruled out free radicals as kinetically important intermediates. At the same time, we developed the instant simple method for synthesis of countless compounds, some of which have never before been synthesized or were synthesized only with great difficulty. Central to our ability to use this system for chemical synthesis was the discovery that this mercury-substrate vapor system could select one among many different reactive molecules, based on the differences in the vapor pressures of these molecules, which phenomenon we call "differential vapor pressure selectivity." No other synthetic technique is known which operates upon this principle. One aspect of this invention is a reactor which takes full advantage of this selectivity.
However, vapor pressure selectivity alone is insufficient to make such mercury photoactivation a commercially attractive synthetic method. The excited mercury atom also exhibits site selectivity upon interaction with an individual substrate molecule. Much work is already in the literature quantifying this selectivity for alkanes as substrates. All investigators found that the order of reactivity for alkane C--H bonds is tertiary&gt;secondary&gt;&gt;primary, although there is great variation in the reported magnitude of the effect. The most definitive study concluded this ratio to be 360:60:1. See Halroyd Klein, J. Phys. Chem, 67, 2277, 1963. Products of simple alkanes of six or fewer carbons were dimers and hydrogen. However, no previous workers discovered or even anticipated that the demonstrated selectivity in the assumed hydrogen abstraction step (Reaction 5) would or could lead to preferential formation at good rates of sterically strained alkanes containing adjacent quaternary carbon centers. For example, one author (H.E. Gunning, J. Chem. Phys. 22, 672, 678, 1954) said that because methylcyclopentane has only one tertiary C--H bond, one would expect that secondary radicals would predominate and that tertiary radicals would be less likely to recombine since steric factors probably would inhibit the formation of the 1,1'dimethylbicyclopentyl. In fact, under the conditions of this invention, over forty percent of the product of this reaction is 1,1'dimethylbicyclopentyl. Although U.S. Pat. No. 2,657,985 describes the dimerization of a 65:35 mixture of propane and isobutane in which the predominant product is 2,2,3,3-tetramethylbutane, and of a 60:40 mixture of isobutane and isopentane in which the predominant product is 2,2,3,3-tetramethylpentane, the only yield which was reported was for the latter which was only 3.0 to 10.1% of the charge, depending on the feed rate of the reactants. In the process of this invention, much higher conversion rates and purer reaction products are obtainable.
Because the process of this invention can be conducted for prolonged periods under simple reflux conditions without adversely affecting the purity of the reaction product, this invention has made photochemical synthesis for the first time an economically feasible method of producing a variety of chemical compounds in commercial amounts. It also permits the production of chemical compounds which could not be produced or could be produced only with great difficulty by chemical synthetic routes.